1.0 Field of the Invention
The present invention relates to microwave couplers and, more particularly, to microwave couplers having means for increasing and varying the range of operating frequencies by means of a voltage control (i.e. voltage tunable). Currently, there is a need for frequency agile materials and components used in a wide variety of applications from communication to radar and electronic countermeasures. This invention contributes to the pursuit of this technology.
2.0 Description of the Prior Art
Microwave couplers are junctions between different sections of transmission lines. They allow microwave radiation to be ducted from one portion of a circuit to another while maintaining amplitude, phase, and modulation integrity. For example, in a microwave transmission line connecting an RF generator to an antenna, a coupler(s) would routinely be placed in the line in order to measure the power being delivered to the antenna. The coupled microwave signal might be only a small fraction of the total power being delivered and might not be at the same phase as the signal transmitted to the antenna, however, these differences should be constant (at a given frequency) and easily characterized. The microwave couplers are fabricated in a predetermined manner so as to provide for a desired center operating frequency with some defined bandwidth, such as fcenter=10 GHz with a 1 GHz bandwidth (i.e., 9.5 to 10.5 GHz). It is desired to provide means that increases the range of operating frequencies for the microwave couplers and, if desired, altering the characteristic impedance of the microwave couplers, while at the same time maintaining or even reducing the insertion loss or the return loss. All this being done while allowing for tunability of the operating frequency of the microwave coupler.
Accordingly, it is a primary object of the present invention to provide means for increasing the range of the operating frequencies of a microwave coupler.
It is another object of the present invention to provide means for altering the characteristic impedance of the microwave coupler.
It is yet another object of the present invention to provide for tunability of the microwave coupler, while at the same time providing a broader range of operating frequencies for the microwave coupler.
It is a further object of the present invention to provide for a tunable branchline coupler, a tunable Wilkinson divider, a tunable backward wave coupler, and tunable Lange couplers each tunable device having increased and adjustable frequency operating ranges.
In accordance with these and other objects, the invention provides for a coupler for transferring a RF signal and having at least one input and at least one output with the input receiving the RF signal. The coupler comprises a transmission line and a piece of ferroelectric material. The transmission line has at least one first section having a first effective length defined by a first quarter wavelength and at least one section serving as the output of the coupler. The piece of ferroelectric material is arranged so as to substitute for a normal substrate material under at least one first section and which alters the first effective length by means of an external d.c. bias so as to be defined by a second quarter wavelength.